Adjustable waveguide termination



Jan. 26, 1960 R. w. BEATTY ADJUSTABLE WAVEGUIDE TERMINATION 2 Sheets-Sheet 1 Filed Aug. 6, 1957 I [IV IN V ENTOR Kabe/"f W fieaf/j k BY M f. WU.

ATTORNEYS Jan. 26, 1960 R. w. BEATTY ADJUSTABLE WAVEGUIDE TERMINATION 2 Sheets-Sheet 2 Filed Aug. 6, 1957 INVENTOR k. .MEEM

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ADJUSTABLE WAVEGUIDE TERMINATION Robert W. Beatty, Boulder, (1010., assignor to the United States of America as represented by the Secretary of Commerce Application August 6, 1957, Serial No. 676,696

Claims. (Cl. 333-22) This invention relates to waveguide terminations and more particularly is concerned with an adjustable sliding termination having provisions for reducing reflections from the termination to a minimum.

Although adjustable waveguide terminations are known to the art, for example, one is disclosed in US. Patent No. 2,701,861 to W. A. Andrews, the present invention provides a termination substantially improving over those of the prior art in several respects.

The termination of the instant invention possesses improved electric stability, easy adjustment and more rugged mechanical construction. It provides a wider usable range of reflection coefficients, may be easily scaled down to smaller waveguide sizes and is simply designed for easy fabrication. Two" principal features of the termination of the present invention are the substantial reduction in over-all length, and the independent variation and locking of three distinct mechanical motions providing substantial improvement in over-all efficiency.

It is therefore one object of this invention to provide an improved adjustable waveguide termination.

Another object of this invention is to provide an adjustable waveguide termination having a wide range of reflection coefficients.

A further object of this invention is to provide an adjustable termination in which the reflection coefficient may be reduced to zero.

A still further object of this invention is to provide a simple, rugged, compact, adjustable Waveguide termination.

Still another object of this invention is to provide an improved waveguide termination having a variable reflection coeflicient which may be selectively positioned along the axis of a waveguide.

Other uses and advantages of the invention will .become apparent upon reference to the specification and drawings, in which:

Fig. 1 shows a diagrammatic view of the adjustable termination of the present invention;

Figs. 2 and 3 show top and side detail views respectively of the waveguide termination of the present invention;

Figs. 46 show top, side, and cross-section detail views respectively of a modified form of the positioning and clamping mechanism of Figs. 2 and 3;

Fig. 7 shows the termination of the present invention incorporated into a directional coupler unit;

Fig. 8 shows a modified embodiment of the resistance strip of Fig. 1, and

Fig. 9 shows an additional modification of the resistance strip of Fig. 1.

In the Andrews patent referred to above reflected termination energy from the combination of a tapered resistance and sliding plunger is cancelled out through the use of a rotatable reflecting obstacle positioned ahead of the termination in the waveguide. Adjustable terminations of this type, however, have the disadvantage that the reflecting obstacle must be supported by a long rod with resulting mechanical instability and increased length of the termination.

United States Patent Q Patented Jan. 26, 1960 The present invention obviates this specific difliculty by eliminating the reflecting obstacle and utilizing the dissipative element itself as means for setting up variable phase, variable amplitudereflections used to cancel out the reflections originating at the plunger.

Referring to the drawings, in Fig. 1 there -is shown-the adjustable termination ofthe present invention. A metallic rod 1 having a square-cross section is rotatably and slidably supported in a sliding tube 2 which tube is in turn connected to a short circuiting plunger 3. Rod 1 is terminated by a dielectric shaft 5 which passes through a central aperture 4 in plunger 3 and extends beyond the face 6 of plunger 3 along the axis of the plunger. Shaft 5 is in turn terminated at its end in a rectangular resistive strip 7 fastened to rod 1 by any suitable fastening means. For example, resistive strip 7 may be glued into a tapered extension of shaft 5 which extension may be slotted to accept the strip. At 8 is shown a section of rectangular waveguide into which the entire assembly is placed to form the adjustable termination of the present invention. As can be seen three separate selective movements are provided by the device of Fig. 1.

The first movement is that of plunger 3 which may he slid longitudinally along the axis of waveguide 8 to provide a short circuit termination which may be positioned at various arbitrary points along the waveguide. The second movement is the sliding movement of resistive strip 7 with respect to plunger 3 occasioned by the longitudinal movement of rod 1 and consequently dielectric shaft 5 through the center of tube 2 and plunger 3. The third movement is the rotary movement of resistive strip 7 brought about by the rotation of rod 1 within tube 2.

The device of Fig. 1 operates as follows. As energy impinges upon the waveguide termination a-certain portion of the energyis reflected by the resistive strip 7 and is transmitted back down the waveguide 8 in the direction from which it came. The remaining portion of the energy incident upon resistive strip 7 passes through the strip with some attenuation and impinges upon the short circuiting plunger 3. This energy is reflected by plunger 3 and passes back through resistive strip 7, again with some attenuation- The relative phase between the reflection at the resistive strip 7 and the reflection at the surface of plunger 3 may be varied by sliding the resistive strip along the waveguide with respect to the plunger. Proper positioning of resistive strip 7 with respect to plunger 3 will result in a displacement in the re-' flected energy from the two elements. With a 180 phase displacement, the reflected energies will tend to cancel each other and a minimum of energy will be reflected from the termination back into the guide.

To provide. perfect cancellation the reflected energies must not only be opposite in phase but they must also be equal in magnitude. The magnitude of the energy reflected from the resistive strip 7 for a given total incident energy only varies within rather small limits as the strip rotates. The reflection coeflicient of the plunger is constant. However, the amount of energy absorbed in passing through the resistive strip 7- can be varied considerably by rotating the rectangular resistive strip with respect to the axis of the waveguide. In this manner the magnitude of the energy reflected from plunger 3 can be selectively controlled and if desired made equal to the energy reflected from strip 7 so that complete cancellation is obtained. In such a situation the'net reflected energy may be zero. Furthermore, by variation of the relative phase of the two reflections and by rotation of the resistive strip a range of reflection coeflicients from zero to 0.9 and higher may be obtained for the terminations r Y 7 Figs. 2 and 3 show top and side views respectively of the entire termination of Fig. 1 and in particular the mechanical control which permits the independent motions referred to above and also show means for locking each motion in the desired position. Metallic rod 1 is mounted in sliding tube 2 by means of a rotating cylindrical collet 11. The cylindrical collet 11 .is suitably slitted to provide jaws adapted to frictionally grip the rod 1. A knob 12 is provided for moving and rotating rectangular rod 1. A first clamping screw 13 acting against cylindrical collet 11 provides means for locking the longitudinal motion of rod 1 by causing the referred-to-jaws to grip rod 11 while a second clamping screw 14 acts against tube 2 to lock rod 1 in the desired rotary position. The position of the plunger and the entire assembly is retained by a third clamping screw 16 acting against the end of waveguide 8 to clamp it to sliding tube 2. Longitudinal movement is imparted to the plunger and the entire assembly by grasping sliding tube 2 to which the plunger is attached. Knob 12 provides means for imparting longitudinal and rotary motion to the resistive strip via rod 1.

As can be seen metallic rod 1 has a square-cross section through a portion of its distance inside tube 2. At the cross-sectional configuration of rod 1 changes from square to circular. The circular portion of rod 1 is recessed at to receive the end of dielectric shaft 5 which may be made of polystyrene or any suitable insulating material.

Figs. 4-6 show top, side, and cross-section views respectively of a modified form of the clamping arrangement shown in Figs. 1-3. Fig. 6 is a cross section taken along lines 6-6 of Fig. 5. The modification of Figs. 4-6 is similar to the clamping arrangement already described but incorporates additional fine adjustments for the rotary and longitudinal positions of rod 1 and hence resistive strip 7. Also in the embodiment shown in Figs. 4-6 the dielectric rod 5 extends through the entire length of the assembly.

Sliding tube 2 as best shown in Fig. 5 is terminated in a lever member 17 having an upstanding U-shaped arm 18. A housing member 19 having a similar upstanding U-shaped arm 20 is coupled for rotary motion to the lever member 17. Each of the arms 18 and 20 support similar washers 21. The arms 18 and 20 and washers 21 are transversely bored to provide passages 22 through which may pass pins (not shown) for supporting the washers in the U-shaped recess in each arm. A fine rotary adjusting screw 23 passes through the washers 21 which may be biased apart by a spring 24.

Housing 19 includes a flange 26 through which passes a fine longitudinal motion adjusting screw 27 supported in the lower portion of a clamp 28. As can be seen if screw 13 is tightened rotation of adjusting screw 27 causes clamp 28 and therefore rod 1 to be drawn toward or away from housing 19 thereby providing a fine adjustment for the longitudinal position of rod 1. Similarly rotation of adjusting screw 23 causes housing 19 to be rotated with respect to stationary lever member 17. Since rod 1 is fixedly engaged to housing 19 by screw 27 it will be rotated in conjunction with housing 19. In this way screw 23 provides a fine adjustment for the rotary position of rod 1. V

Fig. 7 illustrates the use of the variable termination of the present invention in conjunction with a directional coupler. A waveguide directional coupler having a high.

directivity indicated generally at 31 and forming no part of the present invention is used to supply microwave energy from a generator 32 to a measuring circuit or detector 33 indicated in the drawing by blocks labeled generator and detector. Electrical energy from the generator 32 passes through the directional coupler 31 and is largely absorbed by resistance strip 7 of the termination of the present invention. However, a large portion of the energy reflected from the termination back to the directional coupler 31 is coupled through to the detector 33. The directional sensitivity of such a coupler is well known and no more specific description of the coupler operation is deemed necessary. The reading of the detector is an indication of the net reflected energy from the termination. When the termination is adjusted to provide zero reflection a very small amount of energy will flow through the directional coupler to the detector due to the finite directivity of the coupler, but this energy will remain at a constant value and not be affected by sliding the correctly adjusted termination. This use of a directional coupler facilitates rapid adjustment of the termination for minimum reflection, and can also permit a rapid determination of the directivity of the coupler, if desired.

Fig. 8 shows one modification of the rectangular resistive strip 7 of Fig. 1. In this embodiment a reflecting disc 35 is attached to the face of the resistive strip. The addition of disc 35 provides greater reflection at the resistive strip for cancelling out by the reflection from the plunger. With larger cancelling reflections available, less attenuation of the energy passing through the strip is necessary and a shorter resistive strip may be used. The use of a shorter strip provides an embodiment which can be made more mechanically rugged, but which is more frequency sensitive and more diflicult to adjust.

A second modified embodiment of the resistive strip 7 of Fig. l is shown in Fig. 9. As shown in this figure, the resistive strip takes the form of a long-tapered resistive element 36. The long gradual taper reduces the abrupt impedance discontinuity in the waveguide permitting smoother adjustment and is less frequency sensitive. However, the long tapered strip of Fig. 9 is more difficult to fabricate and assemble.

For general use, the unmodified rectangular strip of Fig. 1 represents a most satisfactory attenuating strip and hence is often preferred. with the modification shown in Figs. 8 and 9 having more specialized utility for particular applications.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of invention as defined in the appended claims.

What is claimed is:

1. An adjustable waveguide termination for controlling the reflected energy in a rectangular waveguide comprising: a microwave energy shorting plunger, means positioning said plunger in said waveguide in a direction facing the incident microwave energy applied to said waveguide, said positioning means including means connected to the portion of said plunger obverse to said incident direction for adjustably positioning said plunger longitudinally along the interior of said rectangular waveguide, microwave energy absorbing means, means mounting said absorbing means in said waveguide in a position ahead of said plunger in the direction of said incident energy, said mounting means including means for longitudinally and rotatably selectively positioning said energy absorbing means relative to said shorting plunger.

2. A waveguide termination for insertion into one end of a rectangular waveguide for controlling the reflected energy therein comprising: a shorting plunger, means positioning said plunger in said waveguide in a direction facing the incident microwave energy applied to said waveguide, said positioning means including means connected to the portion of said plunger obverse to said incident direction for adjustably positioning said plunger longitudinally along the interior of said waveguide, microwave energy absorbing means having a noncircular crosssectional configuration, means mounting said microwave energy absorbing means in said'waveguide in a position ahead of said plunger in the direction of said incident energy, said mounting means including means for longitudinally and rotatably selectively positioning said energy absorbing means relative to said shorting plunger.

3. A waveguide termination for controlling the reflected energy in a rectangular waveguide comprising:

elongated waveguide means, microwave energy shorting means, means slidably supporting said shorting means in one end of said waveguide in a direction facing the incident microwave energy in said waveguide, microwave energy absorbing means, means mounting said microwave energy absorbing means in said waveguide in a position displaced from said shorting means along the longitudinal axis of said waveguide means in the direction of said incident energy, said absorbing means having a noncircular, cross-sectional configuration, said mounting means including means for rotating said absorbing means about the longitudinal axis of said waveguide means and means for selectively positioning said absorbing means along the longitudinal axis of said waveguide means relative to said shorting means.

4. A waveguide termination for controlling the reflected energy in a rectangular waveguide comprising: a microwave shorting plunger, means for slidably mounting said plunger in one end of a waveguide in a position facing the incident microwave energy applied to said waveguide, said shorting plunger having a central aperture, tube means connected to the portion of said plunger concentric to said aperture and obverse to said incident direction for imparting longitudinal movement thereto in the direction of the axis of said waveguide, a rod of dielectric material rotatably and slidably supported in said tube means passing through the central aperture in said plunger with the end of said rod extending in said incident direction, and microwave energy absorbing means fixed to said end of said rod.

5. A waveguide termination for controlling the reflected energy in a waveguide comprising: a microwave shorting plunger, means slidably positioning said plunger in one end of a waveguide in a direction facing the incident microwave energy applied to said waveguide, said plunger having a central aperture, tube means connected to the portion of said plunger concentric to said aperture and obverse to said incident direction for imparting longitudinal movement thereto, cylinder means rotatably supported in said tube means, a dielectric rod slidably mounted in said cylinder means for slidable movement 6 with respect thereto, a portion of said rod passing through said aperture with the end of said rod extending in said incident direction, and microwave energy absorbing means on said end of said rod.

6. A waveguide termination for controlling the reflected energy in a waveguide comprising: a microwave shorting plunger, means slidably mounting said plunger in one end of a rectangular waveguide in a position facing the incident microwave energy applied to said waveguide, said plunger having a central aperture, tube means connected to the portion of said plunger concentric to said aperture and obverse to said incident direction for imparting longitudinal movement to said plunger, said tube means communicating with said aperture, a dielectric rod slidably mounted in said tube means and extending through said aperture with the end of said rod extending in said incident direction, cylinder means in said tube means connected to said rod for rotatably and slidably supporting said rod with respect to said plunger and microwave energy absorbing means mounted on said end of said rod.

7. A waveguide termination as defined in claim 6 in which said microwave energy absorbing means comprise an elongated resistive vane.

8. A waveguide termination as defined in claim 7 including a reflecting disc on the end of said vane.

9. A waveguide termination as defined in claim 7 in which said vane is tapered.

10. A waveguide termination as defined in claim 6 in which said securing means includes fine adjustment means for accurately positioning said rod means.

References Cited in the file of this patent UNITED STATES PATENTS 2,530,248 Larson Nov. 14, 1950 2,634,331 Honda Apr. 7, 1953 2,701,861 Andrews Feb. 8, 1955 2,822,517 Cohn Feb. 4, 1958 2,828,469 Martinelli Aug. 20, 1958 

